1. Field of the Invention
The present invention relates to the storage and/or access of digital data. More particularly, the invention concerns a method, apparatus, and article of manufacture using a volume trailer to more efficiently store digital data on a direct access storage device (DASD) in emulation of sequential-access media. Another aspect of the invention concerns the access of data stored pursuant to the invention.
2. Description of the Related Art
Over the years, engineers have developed many different ways of digitally storing data. Two of the most popular involve sequential-access storage drives (i.e., tape media), and DASD. As an example, DASD storage includes magnetic hard disk drives and magnetic floppy diskettes. Tape storage, for instance, includes different electromagnetic or optical wound tapes, and may involve storage patterns such as linear, helical, and serpentine.
DASD and tape storage offer contrasting benefits and limitations. Thus, users may choose DASD for some storage needs, and tape for others. Along these lines, DASD and tape storage have each evolved through the years with contrasting storage formats. This is due to the different physical characteristics of the respective media. In particular, DASD-stored data is randomly accessible, by simply moving a read/write head directly to a specific sector and track location where a desired data item exists. In contrast, tape-stored data is accessed sequentially, by rewinding or advancing the tape until reaching the desired data item.
In tape storage, data is typically stored in units called "records" or "blocks". Interspersed with the records are various headers, each of which contains information describing various characteristics of the associated data record. Adjacent header/record pairs are separated by an inter-block gap (IBG), which is a unique hardware-recognizable pattern of stored data. The IBGs are particularly advantageous when attempting to locate a particular data record on the tape. First, the tape is advanced to a point on the tape where the data record likely begins, or to a point where a group of records including the data record begins. Then, the tape head advances or rewinds, record-by-record, until the desired record is reached. Such movement of the tape head is conducted using the IBGs as guideposts.
In addition to individual record or "block" headers, a tape usually includes a volume header near the beginning of tape (BOT) point. The volume header includes various statistics concerning all records contained on the tape. The volume header is necessarily placed at the tape's beginning because, when a tape cartridge is first loaded to a tape drive, access of the tape begins at the BOT point. To locate the volume header elsewhere would require time consuming forwarding of the tape to reach the volume header.
The foregoing description of tape storage significantly contrasts with DASD storage. Although most formats of DASD storage use headers of various types, DASD storage does not require IBGs. This is because the DASD read/write head, by virtue of its inherent random access capability, can proceed directly to the desired data, without having to sequence record-by-record to find a desired record.
With the advent of virtual tape systems (VTSs), the line between DASD and tape storage has blurred. VTSs chiefly store data on tape, taking advantage of this inexpensive means of long term data storage. However, to expedite data exchanges, data is cached in DASD. According to a predetermined criteria, such as the data's age or recency/frequency of use, the data is backed up on tape. Cache misses result in older data being retrieved from tape and stored again in the DASD cache.
To maintain a consistent data storage format in VTS, it is desirable to use one substantially common tape-like storage format for both DASD and tape media. When this tape-like storage format is applied to DASD, however, some of the normal mechanisms for locating data on tape are no longer useful. Chiefly, the data is not stored with IBGs, which are inapplicable to the direct-access storage format. Even with IBGs, however, access to DASD data would be severely retarded by requiring the DASD head to sequence record-by-record through the data, when direct access is possible. Thus, certain improvements to strict tape storage formats are needed for storing data in the VTS environment, to take advantage of the inherently rapid data access potential of DASD storage.